Consistence and consistency Sadia Bibi Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan 2010
Important from both soil science and engineering point of view
Consistence term is used by soil scientists consistency by engineers
WHAT IS Consistence
Soil consistence is a physical property to describe the resistance of a soil to mechanical stresses or manipulations at various moisture contents
Soil consistence provides a means of describing the degree and kind of cohesion and adhesion between the soil particles as related to the resistance of the soil to deform or rupture.
Since the consistency varies with moisture content and clay minerals, the consistence can be described as dry consistency, moist consistency, and wet consistency.
Consistency that is evaluated includes rupture resistance and stickiness.
The rupture resistance is a field measure of the ability of the soil to withstand an applied stress or pressure as applied using the thumb and forefinger.
Consistence is commonly determined in the field by feeling and manipulating the soil by hand
Observations are made on the amount of force needed to crush the clod and on the manner in which the soil responds to force
The degree of cementation of the soil by such materials as silica, calcite, or iron is also considered in identifying soil consistence
Cohesion is the attraction of one water molecule to another resulting from hydrogen bonding ( water-water bond).
Adhesion is similar to cohesion except with adhesion involves the attraction of a water molecule to a non-water molecule (water-solid bond).
Consistency is used to describe the degree to which a soil resists deformation when a force is applied
Consistency is determined by the soil’s resistance to penetration by an object while the consistence describes resistance to rupture
Instead of crushing a clod of soil, the engineers attempts to penetrate it with either the blunt end of pencil or a thumb nail
Most imp. Property of a soil for engineering uses
This is a measure of the capacity of soil mass to withstand stresses without giving away to those stresses by rupturing or becoming deformed
Soil consistence in general terms can be described as:
Categories are associated with increasing moisture content
Hard or harsh consistency
At low moisture content soil is hard or harsh
If tilled clods formation
As moisture content increases friability (ease of crumbling of moist soils)
For tillage these are optimum conditions
range from firm to loose
As moisture content increases still further stickiness, plasticity, viscous
But when plough under this condition puddling
Air dry consistency
In air dry state consistency is measured by soil’s resistance to rupture or to fragmentation when squeezed. So it is characterized by
Tendency to crush to powder
Inability of crushing material to cohere again when pressed together
Air dry Consistency 1 Newton (N) = 0.224 lb/ft Class Description Loose (sands) Separate from each other when soil is non coherent Symbol L or LO or lo Soft (<8 N) (loams) Soil crushes under very low pressure/ weakly coherent Slightly Hard (8 to < 20 N) Soil material crushes- low pressure - little resistance Mod Hard (20 to < 40 N) Soil material crushes- moderate pressure -resistance Hard (40 to < 80 N) ( clay loam) Soil material crushes under strong pressure Break b/w hands Very Hard (80 to < 160 N) (clays) Can not be crushed between thumb and forefinger but b/w hands with difficulty Extremely Hard Pressure applied by foot with full body or with the help of hammer so depends upon cementation
The capacity of soil to adhere to other objects
Estimated at moisture content that displays maximum adherence between thumb and fore finger
Degree a soil can be molded or reworked causing deformation without rupturing. We take soil material in wet condition and roll it b/w thumb and fore finger and then observe whether rod or wire is formed
Non-Sticky – little or no soil adheres to fingers after release of pressure
Slightly Sticky – soil adheres to one finger but other remain clean after release of pressure with little stretching on separation of fingers
Moderately Sticky – soil adheres to both fingers after release of pressure with some stretching on separation of fingers
Very Sticky - soil adheres firmly to both finger and thumb after release of pressure with stretches greatly on separation of fingers
Non-Sticky Very Sticky Moderately-Sticky
Water Content Significantly affects properties of Silty and Clayey soils (unlike sand and gravel).
Plasticity property describes the response of a soil to change in moisture content.
– Strength decreases as water content increases
– Soils swell-up when water content increases
– Fine-grained soils at very high water content possess properties similar to liquids
– As the water content is reduced, the volume of the soil decreases and the soils become plastic
– If the water content is further reduced, the soil becomes semi-solid when the volume does not change
Non-Plastic – will not form a 6 mm dia, 4 cm long wire, or if formed , can not support itself if held on hand
Slightly Plastic – 6 mm dia, 4 cm long wire supports itself, 4 mm dia, 4 cm long wire does not
Moderately Plastic – 4 mm dia, 4 cm long wire supports itself, 2 mm dia, 4 cm long wire wire does not
Very Plastic – 2 mm dia, 4 cm long wire supports itself
It is a midway b/w air dry consistency and field capacity
It is resistance to squeezing forces b/w thumb and fore finger
Moist Consistency 1 Newton (N) = 0.224 lb/ft Moist comments Stress Loose Non coherent 0 Very Friable Soil material crushes very easily under very gentle pressure < 8 N Friable (best) Soil material crushes under gentle pressure 8 to < 20 N Firm Under moderate pressure b/w thumb and forefinger 20 to < 40 N Very Firm Strong pressure is required 40 to < 80 N Extremely Firm Can’t crush 80 to < 160 N
Behavior of soil at different water contents is called Atterberg limits
Before building roads and motorway is considered
The state of consistency b/w solid and liquid range has been divided into a number of distinct stages by imposing limits to indicate the soil water content limits for various states of consistency.
Depending on the water content of the soil, it may appear in four states:
In each state the consistency and behavior of a soil is different and thus so are its engineering properties. These limits were created by Albert Atterberg , a Swedish chemist. They were later refined by Arthur Casagrande
• Liquid Limit (LL) Upper limit of plasticity of soil
is defined as the moisture content at which soil begins to behave as a liquid material and begins to flow but posses a small shearing strength.
At this point water film becomes so thick that cohesion is decreased and soil mass flows under an applied stress
• Plastic Limit (PL ) is the state of minimum water contents & defined as the moisture content at which soil begins to behave as a plastic material. At that state soil has the property of ploughing. Plastic limit is determined by finding the water contents at which the soil rolled into thread 3mm dia. Begins to break.
• Shrinkage Limit (SL) is defined as the moisture content at which no further volume change occurs with further reduction in moisture content. (SL represents the amount of water required to fully saturate the soil (100% saturation)
Plastic Index (PI)
• Plasticity Index (PI) is the difference between the liquid limit and plastic limit of a soil
By increasing the clay content and O.M. we can increase the plastic and liquid limit
Bigger surface area plasticity more
The knowledge of the soil consistency is important in defining or classifying a soil type
For farming operations e.g., tillage
The soil consistency is a practical and an inexpensive way to distinguish between silts and clays
On organic soils we cant make roads which have high plastic and liquid limits